Energy saving mode with maintained number of advertised transmit antennas

ABSTRACT

Certain aspects of the present disclosure provide techniques for wireless communications, wherein first number of transit antennas is advertised, but a different number of transmit antennas are actually used for transmission.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/357,019, filed on Jun. 21, 2010, which is expressly incorporatedby reference herein in its entirety.

BACKGROUND

I. Field

Certain aspects of the present disclosure generally relate to wirelesscommunications and, more particularly, to a method for disablingtransmit chains without changing an advertised number of transmitantennas.

II. Background

Wireless communication systems are widely deployed to provide varioustypes of communication content such as voice, data, and so on. Thesesystems may be multiple-access systems capable of supportingcommunication with multiple users by sharing the available systemresources (e.g., bandwidth and transmit power). Examples of suchmultiple-access systems include Code Division Multiple Access (CDMA)systems, Time Division Multiple Access (TDMA) systems, FrequencyDivision Multiple Access (FDMA) systems, 3rd Generation PartnershipProject (3GPP) Long Term Evolution (LTE) systems and OrthogonalFrequency Division Multiple Access (OFDMA) systems.

Generally, a wireless multiple-access communication system cansimultaneously support communication for multiple wireless terminals.Each terminal communicates with one or more base stations viatransmissions on the forward and reverse links. The forward link (ordownlink) refers to the communication link from the base stations to theterminals, and the reverse link (or uplink) refers to the communicationlink from the terminals to the base stations. This communication linkmay be established via a single-input single-output, multiple-inputsingle-output or a multiple-input multiple-output (MIMO) system.

A MIMO system employs multiple (NT) transmit antennas and multiple (NR)receive antennas for data transmission. A MIMO channel formed by the NTtransmit and NR receive antennas may be decomposed into NS independentchannels, which are also referred to as spatial channels, where . Eachof the NS independent channels corresponds to a dimension. The MIMOsystem can provide improved performance (e.g., higher throughput and/orgreater reliability) if the additional dimensionalities created by themultiple transmit and receive antennas are utilized.

While considerable focus is placed on conserving power in userequipments (UEs) in order to extend battery life, base stations used inMIMO systems often operate with relatively low efficiency poweramplifiers. As a result, these base stations use relatively high powereven when transmitting only common reference signals (CRS) and notactively serving users.

SUMMARY

Certain aspects of the present disclosure provide a method for wirelesscommunications from a base station. The method generally includessignaling a number of transmit antennas and sending transmissions usinga different number of transmit antennas than signaled.

Certain aspects of the present disclosure provide a method for wirelesscommunications from a base station. The method generally includessending transmissions using a first set of transmit antennas, disablingone or more transmit chains for a corresponding one or more physicalantennas, resulting in a reduced number of active transmit chains,signaling a number of antennas based on the first set of transmitantennas, after the disabling, and sending transmissions using antennasof the reduced number of active transmit chains, wherein the number ofantennas used for transmission with the reduced number of activetransmit chains is less than the signaled number of antennas.

Certain aspects of the present disclosure provide a method for wirelesscommunications from a base station. The method generally includesmonitoring transmissions to determine if a number of transmit antennashas been reduced at a base station by disabling transmit chains andmodifying channel estimation processing functionality, in response todetermining a number of transmit chains have been disabled.

Certain aspects of the present disclosure provide an apparatus forwireless communications from a base station. The apparatus generallyincludes means for signaling a number of transmit antennas and means forsending transmissions using a different number of transmit antennas thansignaled.

Certain aspects of the present disclosure provide an apparatus forwireless communications from a base station. The apparatus generallyincludes means for sending transmissions using a first set of transmitantennas, means for disabling one or more transmit chains for acorresponding one or more physical antennas, resulting in a reducednumber of active transmit chains, signaling a number of antennas basedon the first set of transmit antennas, after the disabling, and meansfor sending transmissions using antennas of the reduced number of activetransmit chains, wherein the number of antennas used for transmissionwith the reduced number of active transmit chains is less than thesignaled number of antennas.

Certain aspects of the present disclosure provide an apparatus forwireless communications from a base station. The apparatus generallyincludes means for monitoring transmissions to determine if a number oftransmit antennas has been reduced at a base station by disablingtransmit chains and means for modifying channel estimation processingfunctionality, in response to determining a number of transmit chainshave been disabled.

Certain aspects of the present disclosure provide an apparatus forwireless communications from a base station. The apparatus generallyincludes at least one processor configured to signal a number oftransmit antennas and send transmissions using a different number oftransmit antennas than signaled; and a memory coupled with the at leastone antenna.

Certain aspects of the present disclosure provide an apparatus forwireless communications from a base station. The apparatus generallyincludes at least one processor configured to send transmissions using afirst set of transmit antennas, disable one or more transmit chains fora corresponding one or more physical antennas, resulting in a reducednumber of active transmit chains, signal a number of antennas based onthe first set of transmit antennas, after the disabling, and sendtransmissions using antennas of the reduced number of active transmitchains, wherein the number of antennas used for transmission with thereduced number of active transmit chains is less than the signalednumber of antennas.

Certain aspects of the present disclosure provide an apparatus forwireless communications from a base station. The apparatus generallyincludes at least one processor configured to monitor transmissions todetermine if a number of transmit antennas has been reduced at a basestation by disabling transmit chains and modifying channel estimationprocessing functionality, in response to determining a number oftransmit chains have been disabled.

Certain aspects of the present disclosure provide a computer-programproduct comprising a computer-readable medium having instructions storedthereon, the instructions executable by one or more processors forsignaling a number of transmit antennas and sending transmissions usinga different number of transmit antennas than signaled; and a memorycoupled with the at least one antenna.

Certain aspects of the present disclosure provide a computer-programproduct comprising a computer-readable medium having instructions storedthereon, the instructions executable by one or more processors forsending transmissions using a first set of transmit antennas, disablingone or more transmit chains for a corresponding one or more physicalantennas, resulting in a reduced number of active transmit chains,signaling a number of antennas based on the first set of transmitantennas, after the disabling, and sending transmissions using antennasof the reduced number of active transmit chains, wherein the number ofantennas used for transmission with the reduced number of activetransmit chains is less than the signaled number of antennas.

Certain aspects of the present disclosure provide a computer-programproduct comprising a computer-readable medium having instructions storedthereon, the instructions executable by one or more processors formonitoring transmissions to determine if a number of transmit antennashas been reduced at a base station by disabling transmit chains andmodifying channel estimation processing functionality, in response todetermining a number of transmit chains have been disabled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication network.

FIG. 2 shows a block diagram of a base station and a UE.

FIG. 3 shows a frame structure for frequency division duplexing (FDD).

FIG. 4 shows two exemplary subframe formats for the downlink.

FIG. 5 shows an exemplary base station and user equipment.

FIG. 6 illustrates example operations that may be performed by a basestation, in accordance with certain aspects of the present disclosure.

FIG. 7 illustrates example operations that may be performed by a userequipment (UE), in accordance with certain aspects of the presentdisclosure.

FIG. 8 illustrates example operations that may be performed by a basestation, in accordance with certain aspects of the present disclosure.

DETAILED DESCRIPTION

Certain aspects of the present disclosure provide techniques that may beutilized to help reduce power consumption by a base station by disablingone or more transmit chains, for example, when traffic demand of UEsserved by the base station is low. Because UEs may not be configured tosupport dynamic changes in a number of transmit antennas used, the basestation may continue to advertise the same number of transmit antennas,even after disabling the one or more transmit chains.

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA), Time Division Synchronous CDMA (TD-SCDMA), andother variants of CDMA. cdma2000 covers IS-2000, IS-95 and IS-856standards. A TDMA network may implement a radio technology such asGlobal System for Mobile Communications (GSM). An OFDMA network mayimplement a radio technology such as Evolved UTRA (E-UTRA), Ultra MobileBroadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20,Flash-OFDM®, etc. UTRA and E-UTRA are part of Universal MobileTelecommunication System (UMTS). 3GPP Long Term Evolution (LTE) andLTE-Advanced (LTE-A), in both frequency division duplexing (FDD) andtime division duplexing (TDD), are new releases of UMTS that use E-UTRA,which employs OFDMA on the downlink and SC-FDMA on the uplink. UTRA,E-UTRA, UMTS, LTE, LTE-A and GSM are described in documents from anorganization named “3rd Generation Partnership Project” (3GPP). cdma2000and UMB are described in documents from an organization named “3rdGeneration Partnership Project 2” (3GPP2). The techniques describedherein may be used for the wireless networks and radio technologiesmentioned above as well as other wireless networks and radiotechnologies. For clarity, certain aspects of the techniques aredescribed below for LTE, and LTE terminology is used in much of thedescription below.

FIG. 1 shows a wireless communication network 100, which may be an LTEnetwork or some other wireless network. Wireless network 100 may includea number of evolved Node Bs (eNBs) 110 and other network entities. AneNB is an entity that communicates with UEs and may also be referred toas a base station, a Node B, an access point, etc. Each eNB may providecommunication coverage for a particular geographic area. In 3GPP, theterm “cell” can refer to a coverage area of an eNB and/or an eNBsubsystem serving this coverage area, depending on the context in whichthe term is used.

An eNB may provide communication coverage for a macro cell, a pico cell,a femto cell, and/or other types of cell. A macro cell may cover arelatively large geographic area (e.g., several kilometers in radius)and may allow unrestricted access by UEs with service subscription. Apico cell may cover a relatively small geographic area and may allowunrestricted access by UEs with service subscription. A femto cell maycover a relatively small geographic area (e.g., a home) and may allowrestricted access by UEs having association with the femto cell (e.g.,UEs in a Closed Subscriber Group (CSG)). An eNB for a macro cell may bereferred to as a macro eNB. An eNB for a pico cell may be referred to asa pico eNB. An eNB for a femto cell may be referred to as a femto eNB ora home eNB (HeNB). In the example shown in FIG. 1, an eNB 110 a may be amacro eNB for a macro cell 102 a, an eNB 110 b may be a pico eNB for apico cell 102 b, and an eNB 110 c may be a femto eNB for a femto cell102 c. An eNB may support one or multiple (e.g., three) cells. The terms“eNB”, “base station” and “cell” may be used interchangeably herein.

Wireless network 100 may also include relay stations. A relay station isan entity that can receive a transmission of data from an upstreamstation (e.g., an eNB or a UE) and send a transmission of the data to adownstream station (e.g., a UE or an eNB). A relay station may also be aUE that can relay transmissions for other UEs. In the example shown inFIG. 1, a relay station 110 d may communicate with macro eNB 110 a and aUE 120 d in order to facilitate communication between eNB 110 a and UE120 d. A relay station may also be referred to as a relay eNB, a relaybase station, a relay, etc.

Wireless network 100 may be a heterogeneous network that includes eNBsof different types, e.g., macro eNBs, pico eNBs, femto eNBs, relay eNBs,etc. These different types of eNBs may have different transmit powerlevels, different coverage areas, and different impact on interferencein wireless network 100. For example, macro eNBs may have a hightransmit power level (e.g., 5 to 40 Watts) whereas pico eNBs, femtoeNBs, and relay eNBs may have lower transmit power levels (e.g., 0.1 to2 Watts).

A network controller 130 may couple to a set of eNBs and may providecoordination and control for these eNBs. Network controller 130 maycommunicate with the eNBs via a backhaul. The eNBs may also communicatewith one another, e.g., directly or indirectly via a wireless orwireline backhaul.

UEs 120 may be dispersed throughout wireless network 100, and each UEmay be stationary or mobile. A UE may also be referred to as a terminal,a mobile station, a subscriber unit, a station, etc. A UE may be acellular phone, a personal digital assistant (PDA), a wireless modem, awireless communication device, a handheld device, a laptop computer, acordless phone, a wireless local loop (WLL) station, a smart phone, anetbook, a smartbook, etc.

FIG. 2 shows a block diagram of a design of base station/eNB 110 and UE120, which may be one of the base stations/eNBs and one of the UEs inFIG. 1. Base station 110 may be equipped with T antennas 234 a through234 t, and UE 120 may be equipped with R antennas 252 a through 252 r,where in general T≧1 and R≧1.

At base station 110, a transmit processor 220 may receive data from adata source 212 for one or more UEs, select one or more modulation andcoding schemes (MCS) for each UE based on CQIs received from the UE,process (e.g., encode and modulate) the data for each UE based on theMCS(s) selected for the UE, and provide data symbols for all UEs.Transmit processor 220 may also process system information (e.g., forSRPI, etc.) and control information (e.g., CQI requests, grants, upperlayer signaling, etc.) and provide overhead symbols and control symbols.Processor 220 may also generate reference symbols for reference signals(e.g., the CRS) and synchronization signals (e.g., the PSS and SSS). Atransmit (TX) multiple-input multiple-output (MIMO) processor 230 mayperform spatial processing (e.g., precoding) on the data symbols, thecontrol symbols, the overhead symbols, and/or the reference symbols, ifapplicable, and may provide T output symbol streams to T modulators(MODs) 232 a through 232 t. Each modulator 232 may process a respectiveoutput symbol stream (e.g., for OFDM, etc.) to obtain an output samplestream. Each modulator 232 may further process (e.g., convert to analog,amplify, filter, and upconvert) the output sample stream to obtain adownlink signal. T downlink signals from modulators 232 a through 232 tmay be transmitted via T antennas 234 a through 234 t, respectively.

At UE 120, antennas 252 a through 252 r may receive the downlink signalsfrom base station 110 and/or other base stations and may providereceived signals to demodulators (DEMODs) 254 a through 254 r,respectively. Each demodulator 254 may condition (e.g., filter, amplify,downconvert, and digitize) its received signal to obtain input samples.Each demodulator 254 may further process the input samples (e.g., forOFDM, etc.) to obtain received symbols. A MIMO detector 256 may obtainreceived symbols from all R demodulators 254 a through 254 r, performMIMO detection on the received symbols if applicable, and providedetected symbols. A receive processor 258 may process (e.g., demodulateand decode) the detected symbols, provide decoded data for UE 120 to adata sink 260, and provide decoded control information and systeminformation to a controller/processor 280. A channel processor 284 maydetermine RSRP, RSSI, RSRQ, CQI, etc., as described below.

On the uplink, at UE 120, a transmit processor 264 may receive andprocess data from a data source 262 and control information (e.g., forreports comprising RSRP, RSSI, RSRQ, CQI, etc.) fromcontroller/processor 280. Processor 264 may also generate referencesymbols for one or more reference signals. The symbols from transmitprocessor 264 may be precoded by a TX MIMO processor 266 if applicable,further processed by modulators 254 a through 254 r (e.g., for SC-FDM,OFDM, etc.), and transmitted to base station 110. At base station 110,the uplink signals from UE 120 and other UEs may be received by antennas234, processed by demodulators 232, detected by a MIMO detector 236 ifapplicable, and further processed by a receive processor 238 to obtaindecoded data and control information sent by UE 120. Processor 238 mayprovide the decoded data to a data sink 239 and the decoded controlinformation to controller/processor 240.

Controllers/processors 240 and 280 may direct the operation at basestation 110 and UE 120, respectively. Processor 240 and/or otherprocessors and modules at base station 110 may perform or directoperations 600 of FIG. 6 and/or other processes for the techniquesdescribed herein. Memories 242 and 282 may store data and program codesfor base station 110 and UE 120, respectively. A scheduler 244 mayschedule UEs for data transmission on the downlink and/or uplink.

As will be described in further detail below, the base station 110 maybe configured to reduce the number of antennas 234 used (by disablingone or more corresponding transmit chains), while still advertising theoriginal (non-reduced) number of transmit antennas. According to certainaspects, the number of antennas may be signaled by scrambling a CRC fora PBCH sent from the base station 110 using a scrambling code dependenton the number of transmit antennas. Thus, the UE 120 may only get asuccessful CRC match if using the original number of transmit antennas,even after the number of transmit antennas has been reduced.

As will be described in greater detail below, according to certainaspects, the UE 120 may determine the actual number of transmit antennasused and modify channel estimation processing functionality accordingly.In this manner, feedback provided by the UE 120, for example, withReference Signal Received Power (RSRP) or Reference Signal ReceivedQuality (RSRQ) measurements, may be compensated for the reduction intransmit antennas and/or certain processing may be disabled to conservepower. According to certain aspects, the base station 110 may select apermutation matrix designed to compensate for the reduction in transmitantennas, for example, by summing virtual antenna port signals andmapping the summed signal to one of the actual transmit antennas used.

FIG. 3 shows an exemplary frame structure 300 for FDD in LTE. Thetransmission timeline for each of the downlink and uplink may bepartitioned into units of radio frames. Each radio frame may have apredetermined duration (e.g., 10 milliseconds (ms)) and may bepartitioned into 10 subframes with indices of 0 through 9. Each subframemay include two slots. Each radio frame may thus include 20 slots withindices of 0 through 19. Each slot may include L symbol periods, e.g.,seven symbol periods for a normal cyclic prefix (as shown in FIG. 2) orsix symbol periods for an extended cyclic prefix. The 2L symbol periodsin each subframe may be assigned indices of 0 through 2L−1.

In LTE, an eNB may transmit a primary synchronization signal (PSS) and asecondary synchronization signal (SSS) on the downlink in the center1.08 MHz of the system bandwidth for each cell supported by the eNB. ThePSS and SSS may be transmitted in symbol periods 6 and 5, respectively,in subframes 0 and 5 of each radio frame with the normal cyclic prefix,as shown in FIG. 3. The PSS and SSS may be used by UEs for cell searchand acquisition. The eNB may transmit a cell-specific reference signal(CRS) across the system bandwidth for each cell supported by the eNB.The CRS may be transmitted in certain symbol periods of each subframeand may be used by the UEs to perform channel estimation, channelquality measurement, and/or other functions. The eNB may also transmit aPhysical Broadcast Channel (PBCH) in symbol periods 0 to 3 in slot 1 ofcertain radio frames. The PBCH may carry some system information. TheeNB may transmit other system information such as System InformationBlocks (SIBs) on a Physical Downlink Shared Channel (PDSCH) in certainsubframes. The eNB may transmit control information/data on a PhysicalDownlink Control Channel (PDCCH) in the first B symbol periods of asubframe, where B may be configurable for each subframe. The eNB maytransmit traffic data and/or other data on the PDSCH in the remainingsymbol periods of each subframe.

FIG. 4 shows two exemplary subframe formats 410 and 420 for the downlinkwith the normal cyclic prefix. The available time frequency resourcesfor the downlink may be partitioned into resource blocks. Each resourceblock may cover 12 subcarriers in one slot and may include a number ofresource elements. Each resource element may cover one subcarrier in onesymbol period and may be used to send one modulation symbol, which maybe a real or complex value.

Subframe format 410 may be used for an eNB equipped with two antennas. ACRS may be transmitted from antennas 0 and 1 in symbol periods 0, 4, 7and 11. A reference signal is a signal that is known a priori by atransmitter and a receiver and may also be referred to as pilot. A CRSis a reference signal that is specific for a cell, e.g., generated basedon a cell identity (ID). In FIG. 4, for a given resource element withlabel R_(a), a modulation symbol may be transmitted on that resourceelement from antenna a, and no modulation symbols may be transmitted onthat resource element from other antennas. Subframe format 420 may beused for an eNB equipped with four antennas. A CRS may be transmittedfrom antennas 0 and 1 in symbol periods 0, 4, 7 and 11 and from antennas2 and 3 in symbol periods 1 and 8. For both subframe formats 410 and420, a CRS may be transmitted on evenly spaced subcarriers, which may bedetermined based on cell ID. Different eNBs may transmit their CRSs onthe same or different subcarriers, depending on their cell IDs. For bothsubframe formats 410 and 420, resource elements not used for the CRS maybe used to transmit data (e.g., traffic data, control data, and/or otherdata).

The PSS, SSS, CRS and PBCH in LTE are described in 3GPP TS 36.211,entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); PhysicalChannels and Modulation,” which is publicly available.

An interlace structure may be used for each of the downlink and uplinkfor FDD in LTE. For example, Q interlaces with indices of 0 through Q−1may be defined, where Q may be equal to 4, 6, 8, 10, or some othervalue. Each interlace may include subframes that are spaced apart by Qframes. In particular, interlace q may include subframes q, q+Q , q+2Q ,etc., where qε{0, . . . Q−1}.

The wireless network may support hybrid automatic retransmission (HARQ)for data transmission on the downlink and uplink. For HARQ, atransmitter (e.g., an eNB) may send one or more transmissions of apacket until the packet is decoded correctly by a receiver (e.g., a UE)or some other termination condition is encountered. For synchronousHARQ, all transmissions of the packet may be sent in subframes of asingle interlace. For asynchronous HARQ, each transmission of the packetmay be sent in any subframe.

A UE may be located within the coverage of multiple eNBs. One of theseeNBs may be selected to serve the UE. The serving eNB may be selectedbased on various criteria such as received signal strength, receivedsignal quality, pathloss, etc. Received signal quality may be quantifiedby a signal-to-noise-and-interference ratio (SINR), or a referencesignal received quality (RSRQ), or some other metric. The UE may operatein a dominant interference scenario in which the UE may observe highinterference from one or more interfering eNBs.

EXAMPLE DISABLING OF TRANSMIT CHAINS

As described above, even when transmitting only CRS, a base station mayoperate a number of relatively inefficient power amplifiers.

A base station may attempt to save energy in various ways. As anexample, a base station may try and save energy by reducing thefrequency of CRS transmissions (e.g., by configuring MBSFN subframes-andnot transmitting on subframes designated as MBSFN), reducing thetransmission bandwidth (BW). Rather than transmit an MBSFN, the basestation may send only pilots in the subframe and may further reducetransmit power consumption by sending only a fraction of pilots.However, is that there is typically a limit in the number of subframesthat can be designated as MBSFN subframes.

According to certain aspects of the present disclosure, the number ofantennas used for transmission may be reduced, reducing the number ofactive transmit chains and reducing power consumption accordingly.According to certain protocols, however, reducing the number of transmitantennas in a dynamic manner may not supported. Therefore, according tocertain aspects, the base station may continue to advertise a previousnumber of transmit antennas, even after reducing the number of activetransmit chains. In other words, the base station may transmit using areduced number of antennas, while advertising a greater number oftransmit antennas.

The techniques presented herein may also be used in coordinatedmulti-point (CoMP) transmission scenarios, for example, when a remoteradio head (RRH) uses a different number of antenna ports when comparedto a donor eNB, but the advertised number of antenna ports has to be thesame because RRH and eNB use the same Cell ID. In other words, whileexamples below refer to reducing the number of transmit chains to reducepower, the techniques may be more broadly applied to any scenario when adifferent number of transmit antennas is advertised than actually usedfor transmission.

FIG. 5 illustrates an example wireless system 500 with a base station(or eNodeB) 510 and UE 520 capable of operating in accordance withcertain aspects of the present disclosure.

According to certain aspects, the base station may dynamically changethe number of transmit antennas used, by enabling/disabling transmitchains of a transmitter module 512. For example, if there are noconnected UEs (e.g., all UEs are in idle mode), the BS 510 may reducetransmit chains to reduce power consumption resulting from transmittingreference signals. As UEs become connected, the BS 510 may re-enable thetransmit chains. As another example, a UE may disable transmit chainswhile a UE has receivers disabled in a discontinuous reception (DRX)mode, and enable the transmit chains in conjunction with the UE enablingits receivers.

As illustrated, the base station 510 may also include a messageprocessing component 514. The message processing component 514 may beconfigured to generate a message, such as a physical broadcast channel(PBCH), to be transmitted to the UE 520, via a transmitter module 512.The base station 510 may signal a fewer number of transmit antennas thanactually used, for example, by continuing to scramble a CRC of the PBCHas if the number of transmit antennas has not changed.

In this example, the UE 520 may receive the PBCH and determine acorresponding number of antennas, based on the CRC. For example, the UE520 may try and decode the PBCH based on a first assumption of transmitantennas (a first hypothesis) and if the checksum fails based on thisfirst assumption, the UE may again decode assuming a different number oftransmit antennas (a different hypothesis). This process may be repeateduntil a checksum succeeds.

As will be described in greater detail below, in some cases, the UE 520may determine an actual number of transmit antennas used (e.g.,different than the number of antennas indicated by the PBCH CRC) bymonitoring for actual transmissions from different antennas. In suchcases, a RRM processing module 524 may take the actual number oftransmit antennas into account when generating feedback. For example,when calculating RSRP and/or RSRQ, rather than average RS signalstransmitted from multiple antennas, since one of the multiple antennasmay not actually be active, the RRM processing module may only considerRS for the actual active antenna.

In any case, the UE 520 may transmit feedback to the BS 510, via atransmitter module 522. The feedback may be received by a receivermodule 516 of the base station 510 and used to adjust transmissionproperties and make various decisions (e.g., regarding scheduling,handover, and the like).

In some cases, the base station 510 may select a particular permutationmatrix for use when sending transmissions using antennas of the reducednumber of active transmit chains. The permutation matrix may be designedto achieve a particular result. As an example, the base station 510 mayselect a permutation matrix that results in summing of at least twovirtual antenna port signals and mapping the summed signal fortransmission via an antenna of an active transmit chain.

Such permutation matrices may be used, for example, when transmitting onone antenna while advertising two antennas or when transmitting on twoantennas while advertising four antennas. This approach may help a UEachieve more accurate measurement reports, in the event the UE does notcompensate, itself, for the reduced number of transmit antennas.

FIG. 6 illustrates example operations 600 that may be performed by abase station (node B) in accordance with certain aspects of the presentdisclosure. For example, the operations 600 may be performed by basestation 510 of FIG. 5.

The operations 600 begin, at 602, by sending transmissions using a firstset of transmit antennas. At 604, the base station may disable one ormore transmit chains, resulting in a reduced number of active transmitchains. At 606, the base station may signal a number of antennas basedon the first set of transmit antennas, after the disabling.

At 608, the base station may send transmissions using antennas of thereduced number of active transmit chains, wherein the number of antennasused for transmission with the reduced number of active transmit chainsis less than the signaled number of antennas.

As noted above, the number of antennas may be signaled by scrambling theCRC of a PBCH based on a scrambling code corresponding to a previouslyused number of transmit antennas. As noted above, the base station mayre-enable transmit chains as conditions change (e.g., UEs becomeconnected, etc.). Further, the base station may also utilize differentpermutation matrices when different numbers of transmit chains areenabled.

While the number of transmit antennas may not change when reducing thenumber of transmit chains, so-called legacy UEs (e.g., that do notsupport dynamic changes in the number of transmit antennas) may stillobserve a change when the number of transmit chains is reduced. Forexample, the UEs may observe a reduction in transmission power (from thedisabled antennas) similar to fading.

In some cases, the observed effect may depend on which transmit chainsare disabled. As an example, when the base station reduces the number ofTx antennas from 4 to 2, there may be no degradation in the SFBCprocessing and there may be no change in the UEs RSRP and RSRQmeasurements. This is because the UE may be mandated not to use thethird and fourth eNB Tx antennas in the RSRP and RSRQ measurements. Whenthe eNB reduces the number of Tx antennas from 2 to 1, there may be nodegradation in the SFBC processing, but there may be inconsistencybetween RSRP and RSRQ measurements from different UEs. This is becausethe UEs are allowed but not mandated to average RSRP over two eNB Txantennas. When the UE is not averaging measurements over the first andsecond eNB Tx antennas, the UE is mandated to use the first eNB Txantenna only for measurements.

To account for this, so-called non-legacy UEs (e.g., LTE Re1-10 UEs) mayimplement an algorithm where a number of received TX antenna portsignals are monitored. In this case, when a Tx antenna port appears tobe absent for an extended period of time, the UE may modify channelestimation processing. For example, related channel estimationprocessing in the UE may be turned off, after which the UE onlyperiodically checks whether the turned off antenna signal reappears. Asanother example, also noted above, a UE may refrain from averaging RSfor multiple antennas when calculating RSRP and/or RSRQ and, instead,use a single antenna RS.

FIG. 7 illustrates example operations 700 a UE may perform to modifychannel estimation based on the actual number of transmit antennas used.The operations 700 may be performed, for example, by UE 520 of FIG. 5.

According to certain aspects, some UEs may monitor transmissions (at702) and determine if a number of transmit antennas used has beenreduced (e.g., despite a different number being advertised). As anexample, a UE may monitor transmissions and determine transmit antennascorresponding to one or more disabled transmit chains are not beingused.

At 704, the UE may modify channel estimation processing functionality inresponse to determining a number of transmit chains has been disabled.As noted above, in an effort to reduce power consumption, the UE maydisable related channel estimation processing functionalitycorresponding to the disabled transmit antennas. The UE may continue tomonitor transmissions to determine if disabled transmit chains have beenre-enabled and, if so, again modify the channel estimation processingfunctionality (e.g., to re-enable previously disabled channel estimationfunctionality or modify how RSRP and/or RSRQ is calculated).

As noted above, the techniques presented herein may be broadly appliedto any scenario when a different number of transmit antennas isadvertised than actually used. FIG. 8 illustrates example operations 800for such a scenario.

The operations 800 begin, at 802, by signaling a number of transmitantennas. At 804, transmissions are sent using a different number oftransmit antennas than signaled. The operations may be performed, forexample, by a remote radio head (RRH) in coordinated multi-point (CoMP)transmissions using a different number of antenna ports when compared toa donor eNB. In some cases, the RRH may not even have as many transmitantennas as advertised.

When the CoMP eNB or RRH uses a smaller number of eNB antenna ports thanadvertised, the same technique as described before can be used forturning off individual transmit chains. In addition, it is also possiblefor a CoMP eNB or RRH to transmit a virtualized transmit signal instead.For example, the sum of signals intended for two Tx antennas can betransmitted on a single antenna, for example. In general, the transmitsignals for N Tx antennas can be transmitted using M physical Txantennas by first multiplying the transmit signals with a M×N linearmatrix. Note that such operation may increase the per tonepeak-to-average value of the transmitted signal; however, due to thenature of the OFDM signal, the time domain peak-to-average power ratiomay not increase. Transmitting virtualized signals may provide onlylimited energy savings but it is useful in the CoMP scenario, especiallywith active connected UEs.

The antenna virtualization technique mentioned above may offer lowerenergy savings due to the fact that multiple pilots still need to betransmitted. Although the power consumption of low power RRH stationsmay not be a concern in general, the eNB may switch between antennavirtualization and antenna turn-off depending on the presence of activeUEs in order to maximize energy saving opportunities.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereofIf implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

1. A method for wireless communications from a base station, comprising:sending transmissions using a first set of transmit antennas; disablingone or more transmit chains for a corresponding one or more physicalantennas, resulting in a reduced number of active transmit chains;signaling a number of antennas based on the first set of transmitantennas, after the disabling; and sending transmissions using antennasof the reduced number of active transmit chains, wherein the number ofantennas used for transmission with the reduced number of activetransmit chains is less than the signaled number of antennas.
 2. Themethod of claim 1, further comprising signaling one or more neighboringbase stations that a different number of transmit antennas is beingadvertised than used for transmission.
 3. The method of claim 1, whereinthe signaling comprises scrambling a checksum based on the number ofantennas.
 4. The method of claim 1, further comprising re-enablingdisabled transmit chains based on a change in one or more conditions. 5.The method of claim 4, wherein the one or more conditions relates to anumber of user equipments (UEs) in a connected mode.
 6. The method ofclaim 4, wherein the one or more conditions relates to a discontinuousreception (DRX) mode of one or more user equipments (UEs).
 7. The methodof claim 1, further comprising: selecting a permutation matrix for usein sending transmissions using antennas of the reduced number of activetransmit chains.
 8. The method of claim 1, wherein the sendingcomprises: utilizing a processing matrix that results in summing of atleast two virtual antenna port signals and transmitting the summedsignal using an antenna of an active transmit chain.
 9. A method forwireless communications by a user equipment, comprising: monitoringtransmissions to determine if a number of transmit antennas has beenreduced at a base station by disabling transmit chains; and modifyingchannel estimation processing functionality, in response to determininga number of transmit chains have been disabled.
 10. The method of claim9, wherein the modifying comprises disabling channel estimationprocessing functionality.
 11. The method of claim 9, further comprising:continuing to monitor transmissions to determine if disabled transmitchains have been re-enabled.
 12. The method of claim 11, furthercomprising: modifying channel estimation processing functionality to aprevious state, in response to determining a number of transmit chainshave been re-enabled.
 13. The method of claim 11, wherein continuing tomonitor transmissions to determine if disabled transmit chains have beenre-enabled comprises periodically monitoring transmissions to determineif disabled transmit chains have been re-enabled.
 14. The method ofclaim 9, wherein the modifying comprises: utilizing reference signalsreceived for fewer than the advertised number of antennas whencalculating one or more receive signal parameter.
 15. A method forwireless communications from a base station, comprising: signaling anumber of transmit antennas; and sending transmissions using a differentnumber of transmit antennas than signaled.
 16. The method of claim 15,wherein the transmissions comprise coordinated multi-point (CoMP)transmissions.
 17. The method of claim 15, wherein the transmissions aresent using a fewer number of transmit antennas than signaled.
 18. Themethod of claim 15, wherein the transmissions are sent using a greaternumber of transmit antennas than signaled.
 19. The method of claim 15,wherein the signaling and sending are performed by a remote radio head(RRH) utilizing a different number of antenna ports than a donor basestation.
 20. An apparatus for wireless communications, comprising: meansfor sending transmissions using a first set of transmit antennas; meansfor disabling one or more transmit chains for a corresponding one ormore physical antennas, resulting in a reduced number of active transmitchains; means for signaling a number of antennas based on the first setof transmit antennas, after the disabling; and means for sendingtransmissions using antennas of the reduced number of active transmitchains, wherein the number of antennas used for transmission with thereduced number of active transmit chains is less than the signalednumber of antennas.
 21. The apparatus of claim 20, further comprisingmeans for signaling one or more neighboring base stations that adifferent number of transmit antennas is being advertised than used fortransmission.
 22. The apparatus of claim 20, wherein the means forsignaling comprises means for scrambling a checksum based on the numberof antennas.
 23. The apparatus of claim 20, further comprising means forre-enabling disabled transmit chains based on a change in one or moreconditions.
 24. The apparatus of claim 23, wherein the one or moreconditions relates to a number of user equipments (UEs) in a connectedmode.
 25. The apparatus of claim 23, wherein the one or more conditionsrelates to a discontinuous reception (DRX) mode of one or more userequipments (UEs).
 26. The apparatus of claim 20, further comprising:means for selecting a permutation matrix for use in sendingtransmissions using antennas of the reduced number of active transmitchains.
 27. The apparatus of claim 26, wherein the means for sendingcomprises: means for utilizing a processing matrix that results insumming of at least two virtual antenna port signals and means fortransmitting the summed signal using an antenna of an active transmitchain.
 28. An apparatus for wireless communications, comprising: meansfor monitoring transmissions to determine if a number of transmitantennas has been reduced at a base station by disabling transmitchains; and means for modifying channel estimation processingfunctionality, in response to determining a number of transmit chainshave been disabled.
 29. The apparatus of claim 28, wherein the means formodifying comprises disabling channel estimation processingfunctionality.
 30. The apparatus of claim 28, further comprising: meansfor continuing to monitor transmissions to determine if disabledtransmit chains have been re-enabled.
 31. The apparatus of claim 30,further comprising: means for modifying channel estimation processingfunctionality to a previous state, in response to determining a numberof transmit chains have been re-enabled.
 32. The apparatus of claim 30,wherein the means for continuing to monitor transmissions to determineif disabled transmit chains have been re-enabled comprises means forperiodically monitoring transmissions to determine if disabled transmitchains have been re-enabled.
 33. The apparatus of claim 28, wherein themeans for modifying comprises: means for utilizing reference signalsreceived for fewer than the advertised number of antennas whencalculating one or more receive signal parameter.
 34. An apparatus forwireless communications, comprising: means for signaling a number oftransmit antennas; and means for sending transmissions using a differentnumber of transmit antennas than signaled.
 35. The apparatus of claim34, wherein the transmissions comprise coordinated multi-point (CoMP)transmissions.
 36. The apparatus of claim 34, wherein the transmissionsare sent using a fewer number of transmit antennas than signaled. 37.The apparatus of claim 34, wherein the transmissions are sent using agreater number of transmit antennas than signaled.
 38. The apparatus ofclaim 34, wherein the apparatus comprises a remote radio head (RRH)utilizing a different number of antenna ports than a donor base station.39. An apparatus for wireless communications, comprising: at least oneprocessor configured to send transmissions using a first set of transmitantennas, disable one or more transmit chains for a corresponding one ormore physical antennas, resulting in a reduced number of active transmitchains, signal a number of antennas based on the first set of transmitantennas, after the disabling, and send transmissions using antennas ofthe reduced number of active transmit chains, wherein the number ofantennas used for transmission with the reduced number of activetransmit chains is less than the signaled number of antennas; and amemory coupled with the at least one processor.
 40. An apparatus forwireless communications, comprising: at least one processor configuredto monitoring transmissions to determine if a number of transmitantennas has been reduced at a base station by disabling transmit chainsand modify channel estimation processing functionality, in response todetermining a number of transmit chains have been disabled; and a memorycoupled with the at least one processor.
 41. An apparatus for wirelesscommunications, comprising: at least one processor configured to signala number of transmit antennas and send transmissions using a differentnumber of transmit antennas than signaled; and a memory coupled with theat least one processor.
 42. A computer-program product comprising acomputer-readable medium having instructions stored thereon, theinstructions executable by one or more processors for: sendingtransmissions using a first set of transmit antennas; disabling one ormore transmit chains for a corresponding one or more physical antennas,resulting in a reduced number of active transmit chains; signaling anumber of antennas based on the first set of transmit antennas, afterthe disabling; and sending transmissions using antennas of the reducednumber of active transmit chains, wherein the number of antennas usedfor transmission with the reduced number of active transmit chains isless than the signaled number of antennas.
 43. A computer-programproduct comprising a computer-readable medium having instructions storedthereon, the instructions executable by one or more processors for:monitoring transmissions to determine if a number of transmit antennashas been reduced at a base station by disabling transmit chains; andmodifying channel estimation processing functionality, in response todetermining a number of transmit chains have been disabled.
 44. Acomputer-program product comprising a computer-readable medium havinginstructions stored thereon, the instructions executable by one or moreprocessors for: signaling a number of transmit antennas; and sendingtransmissions using a different number of transmit antennas thansignaled.